It is known that data-storage optical discs, of the type commercially available under the abbreviation "DVD", ("Digital Video Disc") consist, as regards their structure, of two disc halves each of an approximate 0.6 mm thickness, glued together.
Presently there are different types of "DVD" and, depending on their storage capacity, they are differentiated by abbreviations "DVD5", "DVD9", and "DVD10". "DVD5" consists of a first disc half on which data read by an appropriate laser head is recorded and of a second stiffening disc half. Unlike "DVD5", a "DVD10" is made from two disc halves which already contain data. Data in the second optical disc of a "DVD10" is read by two heads, one located close to one disc face and the other located adjacent to the opposite face. Similarly, "DVD9" is made from two disc halves already containing data but, unlike "DVD10", data is read by means of two laser heads placed at the same surface of the optical disc.
Currently, various processes are used for performing gluing operations in order to achieve finished disks. According to one such current process, an adhesive is spread over the surfaces of the two disc halves which are brought into contact with each other by a roller assembly. More particularly, the disc halves are passed beneath rollers on which an adhesive has been previously distributed so that an even amount of glue is spread over the entire surface to be glued. Subsequently, the two disc halves with the glue distributed thereonto are removed and their glue-wet surfaces are brought into mutual contact so as to thereby obtain a single disc. This disc is then sent to a press where evenness of the glue spread is improved and an efficient gluing is ensured. The disc is then brought to a drying station, generally operating by means of an UV-ray lamp, for drying the glue.
However, this current process described above has been found to be rather complicated when it is incorporated in an automatic production cycle. In the practice of this process, if the two disc halves are ready to be simultaneously joined together, i.e., with glue spread thereon at the same time, two work stations are required to accomplish the rolling operation. From a cost effective standpoint, increased costs are associated with the purchase and maintenance of two identical work stations. Moreover, the apparatus is required to be much more bulky in terms of size and space requirements. In addition, the use of a single inlet station for the rolling operation approximately doubles the time for the gluing step.
In order to overcome the above drawbacks in the automatic production cycle, a further gluing process is also known, in which the two disc halves are moved adjacent to each other and engaged by suction cups or similar means that achieve elastic deformation of the disc halves retaining their central areas in mutual contact and their outer peripheral edges mutually separated from each other. While the disc halves are so arranged, a nozzle of a glue-dispensing device is inserted into the gap created between the disc halves. During this step, the device rotates the two disc halves to turn at least one complete rotation during which glue is released into the central areas of the disc halves. The nozzle is then withdrawn and the two disc halves are released by the suction cups or like means, so that the disc halves resume a flat configuration, thereby achieving glue distribution by capillary action over the entire surface of the disc halves in mutual contact. Optionally, it is possible to rotate the two paired disc halves so that the centrifugal force enables a complete glue spreading.
While this type of technique overcomes some of the inconveniences of the preceding technique, certain drawbacks of this device are also apparent. Foremost, in performing gluing by this process, a certain amount of the distributed glue may escape from the outer peripheral edges of the two disc halves which creates certain problems. As a first consequence o:f the above, a further work station in the optical-disc production cycle becomes necessary in order to eliminate the dried glue present outwardly of the parametric edge of the finished disc, for example, a grinding operation. A further drawback is due to the fact that the volume of glue interposed between the two disc halves cannot be accurately measured.
Moreover, for proper gluing, a glue amount: of about 0.5 ml is used, so as to obtain a nominal thickness of about 50 microns. Therefore, if part of the glue escapes from the discs, it is readily apparent that achieving a precise glue amount and thickness when dealing with such small amounts becomes practically impossible.